1. Field of the Invention
The present invention relates to a circuit board, an electronic device and a method for manufacturing the same.
2. Description of the Related Art
Examples of electronic device include various scales of integrated circuits and various types of semiconductor devices and chips thereof.
As means for realizing a three-dimensional circuit configuration in electronic devices of this type, there has been adopted a method that LSIs are disposed on a circuit board with multilayered wiring therebetween. However, this method increases the mounting area with the number of LSIs, so that the signal delay between LSIs increases because of an increase in wiring length.
Accordingly, there has been proposed a technology of using a circuit board with a circuit pattern disposed on one side and through electrodes passing therethrough in a thickness direction and connected to the circuit pattern.
As one example of such circuit boards, Japanese Unexamined Patent Application Publication No. 11-298138 discloses a process of filling an adhesive liquid material into through-holes or non-through-holes of a multilayered circuit board, wherein after the adhesive liquid material is screen-printed on the circuit board under an vacuum atmosphere, differential-pressure filling is performed by reducing the vacuum degree of the vacuum atmosphere or changing the vacuum atmosphere to a normal atmosphere.
In addition, Japanese Unexamined Patent Application Publication No. 2000-228410 discloses a process of forming through electrodes, wherein through-holes of a high-aspect ratio are formed in a circuit board by an optical excitation electropolishing method, an oxide film as an insulating layer is formed by oxidizing the inner wall of the through-holes, and then the through-holes are filled with a metal by a melted metal refilling process.
Japanese Unexamined Patent Application Publication No. 2002-158191 discloses a process of filling a metal into micropores through a difference in an ambient pressure, while Japanese Unexamined Patent Application Publication No. 2003-257891 discloses a process of filling a conductive paste into micropores. Moreover, Japanese Unexamined Patent Application Publication No. 2006-111896 discloses a process of forming through electrodes, wherein a metal is directly embedded into through-holes before and after a plate embedding stage.
As means for realizing a structure in which through electrodes are connected to a circuit pattern in electronic devices of the type, there are two main types of method. The first method is to previously form through electrodes, which contain Sn (tin) as a main component, on a circuit board and then form a circuit pattern. The second method is to previously form a circuit pattern on a circuit board, perforate through-holes in the circuit board at locations corresponding to the circuit pattern, and then filling a molten metal material such as Sn (tin) into the through-holes for formation of through electrodes.
The circuit pattern is formed by using a thin-film formation technique such as CVD (Chemical Vapor Deposition) or sputtering. Such thin-film formation techniques expose the circuit board to a high temperature. When using the first method, accordingly, the through electrodes containing Sn as a main component melt in the thin-film formation process.
When using the second method, on the other hand, since the through electrodes are formed after formation of the circuit pattern, the problem of melting of the through electrodes in the first method can be avoided. The second method is superior to the first method in that point.
However, although the process of forming the circuit pattern on one side of the circuit board is carried out under a vacuum atmosphere, the circuit board is taken out into the air in the following processes of forming the through-holes and filling the molten metal material thereinto. This results in oxidizing the surface of the circuit pattern at the inner bottom surface of the through-holes.
Oxidation of the surface of the circuit pattern to be connected to the through electrodes deteriorates the connection between the circuit pattern and the through electrodes, thereby causing serious problems that cannot be overlooked in electronic devices of this type, such as insufficient properties and a decrease in yield.
As means for solving the oxidation problem, there has been known a technique of reducing the oxide film of the circuit pattern by using the reduction action of a flux.
However, the above reduction technique using a flux causes the following serious problem. In detail, filling the flux into the through-holes along with the molten metal material generates a flux gas. In electronic devices of this type, the through-holes are micropores having a diameter of, for example, several tens of μm and a significantly high aspect ratio. If a flux gas is generated in such through-holes, the escape of gas inevitably becomes difficult, producing voids due to the flux gas around the through electrodes, which leads to a decrease in cross-sectional area of the through electrodes, an increase in electrical resistance, poor connection to the circuit pattern, and an increase in bond resistance.